Some psychological research has proposed that certain parts of an image
attract our attention more than others [27]. Through visuo-motor
experiments, research has demonstrated that fixation time and attentional
resources are generally allocated to portions in a given scene which are
visually interesting. This ``degree of perceptual significance'' of regions in
a given image allows a human observer to almost automatically discriminate
between insignificant regions in a scene and interesting ones which warrant
further investigation. The ability to rapidly evaluate the level of interest
in parts of a scene could benefit a face recognition system in a similar way:
by reducing its search space for possible human faces. Instead of exhaustively
examining each region in an image for a face-like structure, the system would
only focus computational resources upon perceptually significant objects. It
has been shown that three of the important factors in evaluating perceptual
significance are contrast, symmetry and scale [20].

Neurophysiological experiments demonstrate that the retina performs filtering
which identifies contrast spatially and temporally. For instance, center
surround cells at the retinal processing stage are triggered by local spatial
changes in intensity (contrast) [39]. In psychological tests, humans
detect high contrast objects more readily than, say, objects with a similar
colour to their background. A significant change in spatial intensity is
referred to as an edge, boundary or a contour. Further research has shown that
response to spatial contrast also varies with time [24]. A moving edge,
for example, triggers a strong response at the retinal level [24].
Thus, contrast or intensity changes over space and time are important in
vision and this has lead to the development of edge detectors and motion
detectors.

Another property in estimating perceptual importance is symmetry
[37]. The precise definition of symmetry in the context of
attentional mechanisms is different from the intuitive concept of symmetry.
Symmetry, here, represents the symmetric enclosure or the approximate
encirclement of a region by contours. The appropriate arrangement of edges
which face each other to surround a region attracts the human eye to that
region. Furthermore, the concept of enclosure is different from the
mathematical sense of perfect closure since humans will still perceive a sense
of enclosure despite gaps in boundaries that surround the region
[20].

Scale is also a feature which determines a region's relative importance in a
scene [4]. It is progressively easier to detect a foreground object if
it occupies a greater and greater area in our field of view. Generally, as an
object is enlarged, it increases in relative importance.